1. Field of the Invention
The present invention relates to an apparatus based on the non-dispersive infrared (NDIR) process for the determination of the .sup.13 CO.sub.2 /.sup.12 CO.sub.2 ratio of concentrations in a gas sample.
2. Discussion of the Background
Various substances which are metabolized by the body can be provided with a nonradioactive label, and detected by their metabolic reaction products. One of the essential products of the metabolism of carbon compounds is the CO.sub.2 molecule. If a carbon-containing compound is labelled at a suitable point in the molecule by a .sup.13 C-atom and said compound is administered to a patient, the .sup.13 CO.sub.2 formed therefrom in the patient's body can be detected in the respiratory air. An important use for this technique is in the diagnosis of an infection by Heliobacter pylori, which is the cause of gastric and duodenal ulcers. A .sup.13 C-labelled urea is administered to the patient and .sup.13 CO.sub.2 is formed from it by urease of the bacterium (cf. also R. P. H. Logan et al, European Journal of Gastroenterology & Hepatology 1991, vol. 3, no. 12, pp. 915-921).
Other uses are:
lipase deficiency in pancreatic juices after administering .sup.13 C-labelled triglycerides or triolein; PA1 hepatic cirrhosis or chronic hepatitis after administering .sup.13 C-aminopyrine, .sup.13 C-phanacetin or .sup.13 C-galactose; PA1 absorbtion of harmful substances from the environment, such as PCB after administering .sup.13 C-caffeine.
For all these uses the .sup.13 C-breathing test is a highly appropriate, non-invasive diagnosis method. Due to the very high prices for .sup.13 C-labelled substances and for the necessary detection equipment, these tests have been used almost exclusively in research, and for infants, where the alternatively available biopsy sample tests could not be used. Serological tests offer another alternative but suffer from the disadvantage that they remain positive for a long time after the bacterium has been eradicated.
Investments in facilities for the production of .sup.13 C have taken place at numerous locations and it is expected that .sup.13 C-product prices will drop in the future. However, in order for the respiratory or breathing tests to become attractive, it is also necessary to make available inexpensive detection equipment. The requirements on a measuring process for the detection of .sup.13 CO.sub.2 or for the determination of the .sup.13 CO.sub.2 /.sup.12 CO.sub.2 ratio of concentrations in the respiratory air of a patient are very demanding. For example, following the ingestion of a urea product by a patient infected with Heliobacter pylori, in the period between approximately 5 and 60 minutes after ingesting the product, the isotope ratio rises only from approximately 1.0% (i.e., the normal value corresponding to the natural isotope ratio) to approximately 1.03%. In general, mass spectrometers have been used up to now for detecting this extremely small concentration increase. The very high price of such equipment (approximately 100,000 U.S. dollars) has made it necessary to send the respiratory gas samples to special laboratories having such an apparatus and where the analysis is performed. The resulting logistic and financial costs have constituted a further massive obstacle to widespread use of the .sup.13 CO.sub.2 respiratory gas test.
There is consequently a need for an inexpensive apparatus for .sup.13 CO.sub.2 respiratory gas tests which an ordinary doctor or at least a small laboratory can afford and which permits simple, reliable operation by unspecialized personnel.
A first step in the direction of such an apparatus has recently been taken at the Dusseldorf Laser Medicine Institute by P. Hering and M. Haisch. In the apparatus produced there, which is based on the apparatus according to DE-AI-3522949, pulsed infrared light passes in parallel to, in each case, one cell through which flows a gas sample to be analyzed and is subsequently detected with two optopneumatic receivers. The optopneumatic receivers comprise in each case two gas-filled cells, which are linked to one another by means of a capacitor microphone and whereof one is traversed in each optical path by the infrared light. The capacitor microphones convert the pressure fluctuations produced by the absorbtion of the pulsating infrared light in the particular irradiated cell into electrical signals. For calibrating the apparatus to the normal .sup.13 CO.sub.2 /.sup.12 CO.sub.2 -ratio, special calibrating cells are provided, which are filled with a gaseous mixture having said ratio. They are introduced into the optical paths, but are removed again therefrom for the actual measurement. The amplitudes of the signals supplied by the optopneumatic receivers are compared with one another and the difference of said signals determines the measurement signal. However, quite apart from the fact that this determination requires complicated electronics, this process suffers from the disadvantages that measuring errors, and calibrating errors, and environmental influences enter into the desired measurement result in a highly sensitive manner.